1. Field of the Invention
The present invention relates to a mercury reduction system and a mercury reduction method of flue gas containing mercury that reduce mercury in flue gas discharged from a boiler and the like.
2. Description of the Related Art
Coal combustion flue gas and flue gas generated by burning heavy fuel oil may contain dust, sulfur oxide (SOx), and nitrogen oxide (NOx), as well as metallic mercury)(Hg0). In recent years, various proposals have been made on methods and apparatuses for treating the metallic mercury, by combining a denitration apparatus that reduces NOx and a wet desulfurization apparatus that uses an alkali absorbent as an SOx absorbent.
As a method for treating metallic mercury in flue gas, a system in which NOx in a flue is removed by spraying ammonia (NH3) in the upstream process of a high-temperature denitration apparatus, and mercury is oxidized (chlorinated) on a denitration catalyst by spraying a chlorinating agent such as hydrochloric acid (HCl), turned into an aqueous mercury chloride solution, and removed by a wet desulfurization apparatus installed downstream has been proposed (for example, see Patent Document 1).
FIG. 11 is a schematic of an air pollution control system including a mercury reduction system. As shown in FIG. 11, in this air pollution control system 100 including a mercury reduction system, flue gas 102 including nitrogen oxide and mercury discharged from a boiler 101 is supplied to a reduction denitration apparatus 103, where nitrogen oxide is reduced. The heat of the flue gas 102 is exchanged with air by an air heater 104, and the flue gas 102 is supplied to a dust collector 106, after the heat is collected by a heat collector 105. A desulfurization apparatus 107 reduces sulfur oxide in the flue gas 102, and discharges the flue gas 102 as purified gas 108. The gas is then heated by a reheater 109 and discharged from a stack 110.
An NH3 injection spot 111 is provided upstream of the reduction denitration apparatus 103, and nitrogen oxide is reduced by NH3 supplied from an NH3 tank 112.
A hydrochloric acid measuring unit 113 installed upstream of the desulfurization apparatus 107 in the flue measures the concentration of hydrochloric acid (HCl) used as a mercury chlorinating agent, and a mercury concentration measuring unit 114 installed downstream of the desulfurization apparatus 107 measures the concentration of mercury (Hg). An arithmetic unit 117 calculates an initial concentration of aqueous hydrogen chloride (HCl) solution 116 supplied from a hydrochloric acid solution tank 115, based on the measured concentration values of hydrochloric acid and mercury (Hg). A controlling unit 118 controls the supply of evaporated hydrochloric acid (evaporated HCl) supplied into the flue from the hydrochloric acid solution tank 115 through an HCl injection spot 119.
As a method of spraying HCl from the HCl injection spot 119, a method of reducing mercury combined with a hydrogen chloride (HCl) vaporizer has been proposed (for example, refer to Patent Document 2).
FIG. 12 is a schematic of a mercury chlorinating agent feed apparatus including an HCl vaporizer. As shown in FIG. 12, in a mercury chlorinating agent feed apparatus 120, the aqueous HCl solution 115 at room temperature stored in the hydrochloric acid solution tank 115 is supplied to the side of an HCl vaporizer 122 with a solution feed pump 121, and turned into mixed gas 123 of HCl and water vapor. The mercury chlorinating agent feed apparatus 120 then supplies the mixed gas 123 into a pipe 127 where heated air 126 for dilution supplied from a diluting unit 125 is circulating, through a gas-liquid separator 124, and adjusts to mixed gas 128 of hydrogen chloride/water/air of a predetermined concentration. The temperature of the mixed gas 128 of hydrogen chloride/water/air is normally 70° C. to 80° C. The obtained mixed gas 128 of hydrogen chloride/water/air is dispersed into a flue 139 from the hydrochloric acid injection spot 119, with a disperser 130 having the same configuration as that used for spraying NH3. Accordingly, the mixed gas 128 is uniformly sprayed into the flue gas 102 containing mercury.
As a method of supplying HCl, a method of reducing mercury combined with an apparatus for sublimating ammonium chloride (NH4Cl) solid has been similarly proposed (for example, refer to Patent Document 3).
FIG. 13 is a schematic of an air pollution control apparatus using an NH4Cl solid. As shown in FIG. 13, in an air pollution control apparatus 140, ammonium chloride (NH4Cl) stored in a silo 145 is fed to a crusher 147 by a feeder 146, and added to an economizer bypass unit 144a in powdery form. The economizer bypass unit 144a is placed upstream of a denitration apparatus 141 having an ammonia denitration catalyst, and at an inlet region of an economizer 144 provided in a flue 143 of a boiler facility through which flue gas 142 passes though. NH4Cl is sublimated, due to high ambient temperature of the flue gas 142 of high-temperature (550 to 650° C.) that passes through the economizer bypass unit 144a, and hydrogen chloride (HCl) and ammonia (NH3) are evaporated, and supplied into the flue 143. HCl and NH3 supplied to the flue gas 142 are mixed in the economizer 144.    Patent Document 1: Japanese Patent Publication Laid-open No. H10-230137    Patent Document 2: Japanese Patent Publication Laid-open No. 2007-167743    Patent Document 3: Japanese Patent Publication Laid-open No. 2008-221087
However, in the air pollution control system 100 including a mercury reduction system shown in FIG. 11, and in the mercury chlorinating agent feed apparatus of the mercury reduction system shown in FIG. 12, HCl is a dangerous substance. Accordingly, there poses a problem that extra effort and costs are required to transport and handle HCl.
As the mercury chlorinating agent feed apparatus of the mercury reduction system shown in FIG. 12, if the HCl vaporizer 122 is used, steam 131 and the like is required as a heat source. Accordingly, there poses a problem that installation cost and maintenance cost of the HCl vaporizer 122 and the like are required.
As the air pollution control apparatus shown in FIG. 13, if NH4Cl is used, the size of particles needs to be fine and well dispersed. Accordingly, there poses a problem that the handling of NH4Cl is difficult, and the spraying amount of NH4Cl is not easy to control.
The present invention is made in view of the foregoing, and has an object to provide a mercury reduction system and a mercury reduction method of flue gas containing mercury with enhanced mercury reduction performance and low operational cost.